Electroluminescence display apparatus

ABSTRACT

Side faces of anodes have a tapered incline that becomes broader toward a lower layer. Thus, an emissive element layer is smoothly formed on the anodes making it possible to prevent field contraction of the electric field. An EL display apparatus having long life and high yield is provided by preventing the emissive element layer from rupturing between an anode and a cathode and by preventing concentration of the electric field at an upper edge of the anode facing the cathode and localized deterioration in the emissive element layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.09/679,102, filed Oct. 4, 2000, which is now U.S. Pat. No. 6,727,871,the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a luminescence display apparatuscomprising electroluminescence elements and thin-film transistors.

2. Description of the Related Art

In recent years, electroluminescence (referred to hereinafter as EL)display apparatuses employing EL elements as emissive elements haveattracted attention as being the display apparatuses to replace CRTs andLCDs, and the research and development also have advanced on EL displayapparatuses comprising thin-film transistors (referred to hereinafter asTFT) as switching elements to drive the EL elements.

FIG. 1 shows an equivalent circuit of an EL display apparatus comprisinga conventional EL element and TFT.

FIG. 1 is an equivalent circuit of an EL display apparatus comprising afirst TFT 130, a second TFT 140, and an organic EL element 160, andshows the circuitry near a gate signal line Gn of row n and a drainsignal line Dm of column m.

The gate signal line Gn supplying a gate signal and the drain signalline Dm supplying a drain signal are perpendicular to each other, andnear the intersection of both signal lines are provided the organic ELelement 160 and the TFTs 130, 140 driving the organic EL element 160.

The first TFT 130, which is a switching TFT, comprises gate electrodes131 connected to the gate signal line Gn and supplied with gate signals,a drain electrode 132 connected to a data signal line (drain signalline) Dm and supplied with data signals, and a source electrode 133connected to a gate electrode 141 of the second TFT 140.

The second TFT 140, which is an organic EL element driver TFT, comprisesthe gate electrode 141 connected to the source electrode 133 of thefirst TFT 130, a source electrode 142 connected to an anode 161 of theorganic EL element 160, and a drain electrode 143 connected to a drivingpower supply 150 that is supplied to the organic EL element 160.

Furthermore, the organic EL element 160 comprises the anode 161connected to the source electrode 142, a cathode 162 connected to acommon electrode 164, and an emissive element layer 163 sandwichedbetween the anode 161 and the cathode 162.

Furthermore, a storage capacitor 170 is provided with one electrode 171connected between the source electrode 133 of the first TFT 130 and thegate electrode 141 of the second TFT 140 and another electrode 172connected to a common electrode 173.

The driving method of the circuit shown in the equivalent circuit ofFIG. 1 will now be described. When the gate signal from the gate signalline Gn is applied to the gate electrode 131, the first TFT 130 turnson. As a result, the data signal from the data signal line Dm issupplied to the gate electrode 141 and the voltage of the gate electrode141 becomes identical to the voltage of the data signal line Dm. Acurrent proportional to the voltage value supplied to the gate electrode141 is then supplied from the driving power supply 150 to the organic ELelement 160. As a result, the organic EL element 160 emits light at anintensity in accordance to the magnitude of the data signal.

A conventional EL display apparatus will be described next withreference to FIGS. 2, 3A, and 3B. FIG. 2 is a top view showing one pixelof the conventional EL display apparatus. In FIG. 2, a gate signal line51 corresponds to the gate signal line Gn, a data signal line 52corresponds to the data signal line Dm, a driving power supply 53corresponds to the driving power supply 150, an electrode 54 correspondsto the electrode 172 of the storage capacitor 170, and an anode 61corresponds to the anode 161 of the organic EL element 160. The gatesignal lines 51 are arranged in rows and the data signal lines 52 andthe driving supplies 53 are arranged in columns. The storage capacitorand the emissive element layer are arranged within the area thuspartitioned. The storage capacitor is formed from a semiconductor film13 and the electrode 54. The semiconductor film 13 is connected to thedata signal line 52 via a contact C1, and a gate electrode 11 isarranged between a drain 13 d and a source 13 s.

A semiconductor film 43 is connected to the driving power supply 53 viaa contact C2, and a gate electrode 41, which is connected to thesemiconductor film 13, is arranged between a drain 43 d and a source 43s. The semiconductor film 43 is connected to the anode 61 of the organicEL element via a contact C3.

FIG. 3A is a cross-sectional view along line A—A of FIG. 2. On atransparent substrate 10 is formed the semiconductor film 13, on whichis covered with and formed a gate insulating film 12. On the gateinsulating film 12 are formed gate electrodes 11, which branch from thegate signal line 51, and the storage capacitor electrode 54, on which iscovered with and formed an interlayer insulating film 15. On theinterlayer insulating film 15 is arranged the data signal line 52, whichconnects to the semiconductor film 13 via the contact C1. On these iscovered with and formed a planarization insulating film 17.

FIG. 3B is a cross-sectional view along line B—B of FIG. 1. On thesubstrate 10 are laminated in sequence the semiconductor film 43, thegate insulating film 12, the gate electrode 41, and the interlayerinsulating film 15, and on the interlayer insulating film 15 are formedthe data signal line 52 and the driving power supply 53, on which iscovered with and formed the planarization insulating film 17. On theplanarization insulating film 17 is arranged an anode 61, which isconnected to the semiconductor film 43 via the contact C3. On the anode61 is arranged an emissive element layer 66, which has a laminatedstructure of a first hole transport layer 62, a second hole transportlayer 63, an emissive layer 64, and an electron transport layer 65. Acathode 67 is arranged so as to cover them.

The anode 61 of the pattern shown in FIG. 2 is generally formed using amethod in which an ITO film is first formed on the entire surface, andafter forming a positive photoresist in a predetermined shape, wetetching is performed using chemicals.

However, when forming the organic EL element in this manner, theemissive element layer 66 that is formed on the anode 61 is extremelythin at approximately 200 nm so that coverage at the step portion withthe planarization insulating film 17 at the edge of the anode 61deteriorates. Thus, at the points indicated by the arrows in FIG. 4,since the vertex of the anode 61 and the vertex of the cathode 67 faceeach other in closer proximity than at any other location, fieldconcentration occurs here causing a problem where the emissive layer 64positioned between layers deteriorates rapidly. As the coveragedeteriorates further, the emissive element layer 66 ruptures as shown inthe figure, and the cathode 67 provided on the upper layer shorts withthe anode 61 to possibly cause this pixel to be defective and notdisplay.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an ELdisplay apparatus having long life and high yield by preventing shortsor localized deterioration of the emissive layer 64 due to the thicknessof the anode.

The present invention solves the aforementioned problem and is anelectroluminescence display apparatus comprising an emissive element (anelectroluminescence element) laminated in sequence on the substrate withthe first electrode, the emissive element layer (such as hole transportlayer, emissive layer, and electron transport layer), and the secondelectrode, with the side faces of the first electrode inclined andbecoming broader toward the substrate side.

The angle formed by the incline of the first electrode and the plane ofthe lower layer (and/or the substrate) is 10° to 45°, or further anangle of 25° to 35°. Furthermore, the side of the first electrode has atapered shape becoming broader from the emissive element layer towardthe substrate.

Furthermore, the thickness of the first electrode is less than ½, orfurther less than ⅓ the total film thickness of the hole transportlayer, the emissive layer, and the electron transport layer.

As described above, the edge of the first electrode in the presentinvention is inclined so that the electroluminescence element that isformed thereon is formed smoothly, shorting of the first electrode andthe second electrode is prevented, and an electroluminescence displayapparatus having a high yield is obtained.

Furthermore, since concentration of the electric field at the edge ofthe first electrode is prevented, the electroluminescence element isprevented from locally deteriorating, and a luminescence displayapparatus having a long life is obtained.

Since the angle formed by the incline of the first electrode with theplane of the lower layer (and/or the substrate) is 10° to 45°, orfurther an angle of 25° to 35°, this ensures the emissive element layercan be formed without loss of reproducibility of the shape of the firstelectrode.

Furthermore, since the thickness of the first electrode is less than ½,or further less than ⅓, the film thickness of the emissive elementlayer, this ensures the emissive element layer can be formed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an equivalent circuit diagram of an EL display apparatus.

FIG. 2 is a cross-sectional view of the EL display apparatus of thepresent invention.

FIGS. 3A and 3B are cross-sectional views of the EL display apparatus ofthe present invention.

FIG. 4 is a cross-sectional view illustrating a problem of aconventional EL display apparatus.

FIG. 5 is a top view of an active-matrix EL display apparatus of thepresent invention.

FIG. 6 is a cross-sectional view of the active-matrix EL displayapparatus of the present invention.

FIGS. 7A and 7B are enlarged cross-sectional views showing the edge ofthe first electrode of the present invention.

FIGS. 8A, 8B, and 8C are cross-sectional views showing a formationmethod of the first electrode of the present invention.

FIG. 9 is a top view and a cross-sectional view of a simple-matrix ELdisplay apparatus of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first embodiment of the present invention will be describedhereinafter. The first embodiment is an example applying the presentinvention to an active-matrix organic EL display apparatus. One displaypixel of the EL display apparatus of the first embodiment is shown inFIG. 5 and a cross-sectional view along line A—A in FIG. 5 is shown inFIG. 6.

A driver circuit for each pixel is identical to the circuit shown inFIG. 1, and the difference with the prior art shown in FIGS. 2, 3A, and3B is the cross-sectional configuration of an anode 1, or firstelectrode.

The gate signal line 51, the data signal line 52, the driving powersupply 53, the electrode 54, and the anode 1 respectively correspond tothe gate signal line Gn, the data signal line Dm, the driving powersupply 150, the electrode 172 of the storage capacitor 170, and theanode 161 of the organic EL element 160. The gate signal lines 51 arearranged in rows and the data signal lines 52 and the driving supplies53 are arranged in columns. A capacitor and an emissive layer arearranged within the area that is partitioned by the signal lines andpower supply lines. The storage capacitor is formed from thesemiconductor film 13 and the electrode 54. The semiconductor film 13 isconnected to the data signal line 52 via the contact C1, and the gateelectrode 11 is arranged between the drain 13 d and the source 13 s.

The semiconductor film 43 is connected to the driving power supply 53via the contact C2, and the gate electrode 41, which is connected to thesemiconductor film 13, is arranged between the drain 43 d and the source43 s. The semiconductor film 43 is connected to the anode 1 of theorganic EL element via the contact C3.

As shown in FIG. 6, the organic EL display apparatus is formed bylaminating in sequence a TFT and an organic EL element on the substrate10, such as a substrate formed from glass or synthetic resin, aconductive substrate, or a semi-conductive substrate. However, when aconductive substrate or a semi-conductive substrate is used for thesubstrate 10, an insulating film of SiO₂ or SiN is formed on thesubstrate 10, upon which the TFT and organic EL element are formed.

In the present embodiment, a first TFT 30 and a second TFT 40 are bothso-called top-gate TFTs provided with a gate electrode at the top of theactive layer, and a case is given where a semiconductor film formed frompoly-silicon is used for the active layer. Furthermore, the case isgiven where the TFT has the gate electrode 11 with a double-gatestructure.

The first TFT 30, which is a switching TFT, will be described first.

As shown in FIG. 6, on the insulating substrate 10, which is formed fromquartz glass, non-alkaline glass, or the like, are formed in sequencethe semiconductor film 43 and the gate insulating film 12. Thesemiconductor film 43 is the active layer of the second TFT, and has thesource 43 s, the drain 43 d, and the channel 43 c. On the gateinsulating film 12 is formed the gate electrode 41, which is formed froma refractory metal such as chromium (Cr), molybdenum (Mo), or the like,on which is covered with and formed the interlayer insulating film 15,which is formed by laminating in sequence a SiO₂ film, a SiN film, and aSiO₂ film. Thereon is formed the data signal line 52 and the drivingpower supply 53.

The TFT has a so-called Lightly Doped Drain (LDD) structure. Namely, iondoping is performed using the gate electrode 41 on a channel 13 c as amask. Furthermore, the gate electrode 41 and an area up to a fixeddistance from both sides of the gate electrode 41 are covered withresist, and ion doping is performed again to provide a low concentrationarea on both sides of the gate electrode 41 and beyond these areas thesource 43 s and the drain 43 d of a high concentration area.

Furthermore, the planarization insulating film 17, which is formed froman organic resin or the like, is formed on the entire surface so as toplanarize the surface. A contact hole is then formed at a positioncorresponding to the source 43 s in the planarization insulating film17, and a transparent first electrode formed from ITO and contacting thesource 43 s via the contact C3, namely, the anode 1 of the organic ELelement, is formed on the planarization insulating film 17.

The emissive element layer 66 adopts a common structure and is formed bylaminating in sequence the anode 1 formed from a transparent electrode,such as ITO, the first hole transport layer 62 formed from MTDATA(4,4′,4″-tris (3-methylphenylphenylamino)triphenylamine), the secondhole transport layer 63 formed from TPD (N,N′-diphenyl-N,N′-di(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine), the emissive layer 64formed from Bebq₂ (bis(10-hydroxybenzo[h]quinolinato) beryllium)including an inductor Quinacridon, the electron transport layer 65formed from Bebq₂, and the cathode 67 formed from a magnesium-indiumalloy or a magnesium-silver alloy or a lithium fluoride-aluminumlamination.

Furthermore, in the organic EL element, holes injected from the anodeand electrons injected from the cathode recombine within the emissivelayer, and the organic molecules included in the emissive layer areexcited to yield exitons. Light is released from the emissive layer inthe process where the exitons undergo radiation deactivation, and thislight is released to the outside from the transparent anode via thetransparent insulating substrate to make the light emission visible.

Arranging display pixels configured in this manner in a matrix on thesubstrate 10 forms an organic EL display apparatus capable of displayinga desired overall image by controlling each pixel.

The anode 1 of the present embodiment has edges forming tapered inclinesas shown in FIG. 6. Due to these inclines, the emissive element layer 66is smoothly formed from the anode 1 on the planarization insulatinglayer 17, thereby preventing the coverage from deteriorating and theanode 1 and the cathode 67 from shorting. Furthermore, since theinclines become broader on the substrate side, there are no sharp edgeson the top edge of the anode 1 facing the cathode 67, making fieldconcentration less likely to occur. Therefore, the emissive layer 64emits light uniformly on the entire surface and partial deteriorationdoes not occur rapidly.

It is preferable for the angle θ formed from the incline of the anode 1shown in FIGS. 7A and 7B with the plane of the planarization insulatingfilm 17 to be small so as to prevent rupture or field concentration.However, if the angle θ is too small, the edge of the anode 1 becomesextremely thin so that a problem arises where reproducibility of theshape decreases. Therefore, the angle formed by the plane of the bottomlayer or the substrate 10 with the inclined side faces of the anode 1 isset from 10° to 45°, and preferably around 30°. Furthermore, it ispreferable for the top edge of the anode 1 to have a smooth curve asshown in FIG. 7B.

A method for forming the anode 1 into an incline will be described next.As described above, although the etching of the ITO film employed theconventional wet etch method, the angle θ of the incline becomessubstantially 90°. In the present embodiment, a positive photoresist isformed on the ITO film, which has been formed on the entire surface, anddry etching is performed using a chlorine-based gas, such as Cl₂ or HCl,to form an incline on the ITO edge. FIGS. 8A to 8C are cross-sectionalviews showing the formation method of the anode 1. First, as shown inFIG. 8A, an ITO film 21 is formed on the entire surface of theplanarization insulating film 17. Next, a positive photoresist 22 isformed at a predetermined area. When this is exposed to a chlorine-basedgas, such as chlorine gas or hydrogen chloride gas, the ITO film 21 andthe photoresist 22 are etched isotropically. With dry etching usingchlorine-based gas, the selectivity is low between the ITO film 21 andthe photoresist 22 so that the photoresist 22 is etched simultaneouslywith the ITO film 21. However, since etching is faster for the ITO film21, the etching proceeds as shown in FIG. 8B even though the selectivityis low. The etching continues and completes as shown in FIG. 8C. In thepresent embodiment, the angle θ of the incline becomes approximately30°. In this manner, the isotropic etching is performed using an etchinggas having low selectivity between the ITO film and the resist so thatthe anode 1 is formed with sloping edges.

The film thickness of the anode 1 is described next. The film thicknessof the anode 1 is thinly formed compared to the total film thickness ofthe emissive element layer 66. When the film thickness of the anode 1 isthin, a step developing between it and the planarization insulating film17 is reduced so that a rupture of the emissive element layer 66 can beprevented. Since the color of the display changes depending on thethickness of the anode 1, an arbitrary thickness cannot necessarily beset. The film thickness of the anode 1 is set to ½ the total thicknessof the emissive element layer 66 or less if possible, and preferably to⅓ or less. However, if the anode 1 is formed too thin, thereproducibility of the shape decreases due to chipping of part of theanode 1 and so forth. In the present embodiment, the anode 61 has athickness of approximately 85 nm, the emissive element layer 66 has atotal thickness of approximately 200 nm, and the cathode 67 has athickness of approximately 200 nm.

The present invention is also applicable to a simple-matrix EL displayapparatus. FIG. 9 shows a top view and a cross-sectional view along lineA—A of the simple-matrix EL display apparatus representing a secondembodiment of the present invention.

Arranged on a transparent substrate 70 are an anode 71, which is a firstelectrode extending longitudinally, and a cathode 72, which is a secondelectrode extending transversely and crossing the first electrode 71. Inthe emissive element layer 66, the emissive layer 64 is formed at eachintersection of the anode 71 and the cathode 72.

Although the TFT was illustrated in the aforementioned embodiments ashaving the top-gate structure in which the gate electrode is located onthe active layer, it may have a bottom-gate structure instead.Furthermore, although a semiconductor film was used for the active layerin the aforementioned embodiments, a micro-crystalline silicon film oramorphous silicon may be used instead.

In this embodiment also, the edge of the anode 71 inclines and becomesbroader toward the substrate so that the emissive element layer 66smoothly covers the anode 71, thereby preventing shorts between theanode 71 and the cathode 72.

Furthermore, although an organic EL display apparatus was described inthe aforementioned embodiments, the present invention is not limitedthereto, and may be also applicable to an inorganic EL display apparatushaving a emissive layer formed from inorganic materials, while yieldinga similar effect.

Furthermore, although the first electrode was described in the presentspecification as an anode, the first electrode is arranged between thesubstrate and the EL element (EL layer) and is the electrode covered bythe EL layer so that in some cases it may be a cathode.

While there has been described what are at present considered to bepreferred embodiments of the invention, it will be understood thatvarious modifications may be made thereto, and it is intended that theappended claims cover all such modifications as fall within the truespirit and scope of the invention.

1. An electroluminescence display apparatus comprising: a firstelectrode formed above a substrate; an emissive element layer formed onsaid first electrode; a second electrode formed on said emissive elementlayer; and a thickness of said first electrode is less than ½ athickness of said emissive element layer, said thickness of saidemissive element layer is approximately 200 nm.
 2. Anelectroluminescence display apparatus comprising: a first electrodeformed above a substrate; an emissive element layer formed on said firstelectrode; a second electrode formed on said emissive element layer; anda thickness of said first electrode is less than ⅓ a thickness of saidemissive element layer, said thickness of said emissive element layer isapproximately 200 nm.
 3. An electroluminescence display apparatuscomprising: a first electrode formed above a substrate; an emissiveelement layer formed on said first electrode; a second electrode formedon said emissive element layer; and a thickness of said first electrodeis less than ½ a thickness of said emissive element layer, saidThickness of said emissive element layer is approximately 200 nm,wherein said electroluminescence display apparatus is an active-matrixtype comprising said first electrode formed independently at each pixel,and thin-film transistor for driving said emissive element.
 4. Anelectroluminescence display apparatus according to claim 3 furthercomprising the planarization insulating film formed so as to cover saidthin-film transistor, with said first electrode formed on saidplanarization insulating film.
 5. An electroluminescence displayapparatus according to claim 3 wherein said emissive element layercomprises a layered structure of a hole transport layer, an emissivelayer, and an electron transport layer.
 6. An electroluminescencedisplay apparatus comprising: a first electrode formed above asubstrate; an emissive element layer formed on said first electrode; asecond electrode formed on said emissive element layer; and a thicknessof said first electrode is less than ½ a thickness of said emissiveelement layer, said thickness of said emissive element layer isapproximately 200 nm, wherein said electroluminescence display apparatusis a passive-matrix type wherein said first electrode extends in a firstdirection and said second electrode extends in a second direction so asto intersect said first electrode.
 7. An electroluminescence displayapparatus according to claim 6 wherein said emissive element layercomprises a layered structure of a hole transport layer, an emissivelayer, and an electron transport layer.
 8. An electroluminescencedisplay apparatus comprising: a first electrode formed above asubstrate; an emissive element layer formed on said first electrode, theemissive element layer comprises an organic layer that includes at leastorganic emissive molecules; a second electrode formed on said emissiveelement layer; a thickness of said first electrode is less than ½ thethickness of said emissive element layer; and wherein said firstelectrode is formed independently at each pixel; each pixel comprises athin film transistor for driving said emissive element layer; and aplanarization insulating film is formed so as to cover said thin filmtransistor, with said first electrode being formed on said planarizationinsulating film.
 9. An electroluminescence display apparatus comprising:a first electrode formed above a substrate; an emissive element layerformed on said first electrode, the emissive element layer comprises anorganic layer that includes at least organic emissive molecules; asecond electrode formed on said emissive element layer; a thickness ofsaid first electrode is less than ⅓ a thickness of said emissive elementlayer; and wherein said first electrode is formed independently at eachpixel; each nixel comprises a thin film transistor for driving saidemissive element layer; and a planarization insulatina film is formed soas to cover said thin film transistor, with said first electrode beingformed on said planarization insulating film.